Desulfation of such a trap is essential to maintain a lasting NOx conversion activity, due essentially to its progressive poisoning by the sulfur contained in the fuel and carried by the exhaust gas flowing therethrough. This phenomenon takes place whatever the sulfur content of the fuel, even with sulfur contents below 10 ppm.
To carry out the desulfation operations, it is necessary to heat the trap to a temperature higher than a predetermined temperature (generally above 600° C.) for about 10 minutes, generally by means of an exhaust gas temperature increase, and to have a composition that is globally reducing for this gas so as to allow conversion of the sulfur accumulated in the trap and thus to restore its conversion activity.
This exhaust line can also comprise a particle filter whose function consists in holding back the particles present in the exhaust gas and thus to prevent discharge thereof into the atmosphere.
This filter also has to be periodically regenerated in order to keep all its filtration capacities. The regeneration operations consist in increasing the temperature of the filter to about 450° C., generally by increasing the fuel/air ratio of the exhaust gas flowing therethrough without it reaching fuel/air ratio 1, and in obtaining an oxidizing composition for this gas so as to achieve combustion of the particles kept in the filter.
Document FR-2,825,412 describes a method intended for desulfation of a NOx trap and regeneration of a particle filter.
In this document, an internal-combustion engine comprises an exhaust gas cleaning system with a NOx trap arranged upstream from a particle filter. By means of the method described, the fouling condition of the particle filter, or clogging rate, is constantly monitored and the particle filter regeneration operations are started as soon as the clogging rate has exceeded a certain threshold. During these regeneration operations, the state of sulfur saturation of the trap, or saturation rate, is examined and, if this rate has reached a predetermined value, the NOx trap desulfation operations are started at the end of the regeneration operations. In the opposite case, only the filter regeneration operations are carried out and the trap continues to accumulate the sulfur contained in the exhaust gas.
These desulfation and regeneration operations however involve quite significant drawbacks.
Notably, the particle filter efficiency can deteriorate locally and durably as a result of an excessive temperature during regeneration, leading to degradation of the filter materials, a phenomenon that occurs especially in the central part thereof. Conversely, the external part of the filter can suffer from an insufficient temperature rise, because of its proximity to the walls and of the inhomogeneous nature of the gas stream flowing through the filter, and may not be entirely freed of the particles accumulated during regeneration.
Furthermore, it has been observed that the NOx trap loses efficiency after a certain number of desulfation operations. In fact, the internal part of the grains of the storage materials that make up the trap is not very accessible to the exhaust gas during the desulfation operations, and sulfates with a very stable crystal structure at high temperatures, of the order of 900° C., form and put a definitive end to the conversion activity of these grains. To maintain a minimum conversion efficiency required and to fulfil the endurance objectives of the depollution system imposed by regulations, it is necessary to increase the frequency of the desulfation operations as the vehicle travels miles. This frequency increases further if the driving mode is often a high-load driving mode. Such a mode is often accompanied by a high fuel consumption which generates an increasing sulfur accumulation in the trap. This accumulation increase leads to an increase in the frequency of the desulfation operations, which involves a quite significant fuel consumption increase insofar as these operations require an exhaust gas fuel/air ratio above 1, of the order of 1.1, and an exhaust gas temperature above 600° C. Furthermore, passage to this fuel/air ratio and to this high temperature leads to a high combustion degradation in the engine because the air has to be throttled at the intake and the amount of fuel injected into the combustion chamber of the engine has to be increased.
Another drawback of desulfation is the discharge of sulfur compounds, notably H2S, throughout the desulfation operations. This H2S discharge is malodorous and, at high concentration, it can be dangerous to man's health.
It has also been observed that, according to a major drawback of the method described in the aforementioned document, the NOx trap can be saturated long before the particle filter regeneration operations are started. The trap can therefore no longer fulfil its function and the sulfur compounds contained in the exhaust gas will be discharged into the open air without being treated.
The present invention aims to overcome the aforementioned drawbacks by means of a method intended for desulfation of a NOx trap and regeneration of a particle filter that does not penalize the fuel consumption and allows the NOx trap efficiency to be kept at a very high level throughout the life of the vehicle.